Generally, steel sheets for applications such as automobile panels are treated with phosphates and are painted after a car body assembly process in an automotive manufacturing plant. In this case, several folded portions may be present in steel sheets of an assembled car body. Since it is difficult for phosphates or paint to permeate into the folded portions, the folded portions may be vulnerable to corrosion.
In the related art, a method of sealing the folded portions of steel sheets has been used to prevent corrosion-causing substances from permeating into the folded portions. However, since the method decreases the productivity of final products and increases manufacturing costs, surface-treated steel sheets having high corrosion resistance and thus not requiring sealing treatments have been in demand.
Recently, steel sheets formed by coating base steel sheets with organic films have been widely researched as types of surface-treated steel sheets not requiring sealing treatments. As a result, surface-treated steel sheets coated with organic films of a uniform thickness are being commercialized.
Such surface-treated steel sheets coated with the organic films ensure corrosion resistance even in portions thereof which are not painted or treated with phosphate, but the weldability thereof during electric resistance welding may be lowered due to the organic films being relatively thick.
Therefore, a surface-treated steel sheet having high corrosion resistance and excellent weldability during electric resistance welding is required.